Since narrow frequency band communication signals are easily detected, jammed, etc., military communication systems utilize bandspread communication signals. However, in a military theater of operation, unfriendly forces intentionally interject non-Gaussian interference into the radio transmission spectrum which can interfere with the reception of even bandspread signals. Additionally, such "intentional" interference generally dominates the transmission signal as well as any Gaussian noise associated therewith.
One type of such interference is known as "on-off" interference. On-off interference is generally caused by two narrow band interferers radiating from separate interference sources. The interference pattern from the two sources interfere with one another such that they add together when in phase and cancel each other when out of phase. The resulting interference appears as a pulsed interference whereby the amplitude of any given pulse is approximately linear over the length of the pulse.
Prior art on-off interference suppression systems have typically applied adaptive filtering techniques to suppress interference from bandspread communication signals. One of the most effective techniques to date involves the use of nonlinear transformations of received signal samples. These transformations have traditionally been obtained by the estimation of the probability density function. The derivatives necessary to obtain the transformation from the probability density function have been calculated by using finite difference operations. However, this approach is computationally demanding and can only be implemented through the use of a dedicated microprocessor. In addition, this approach is only useful for digital radios operating in the lower portion of the radio frequency spectrum (i.e., LF, VLF or ELF) due to sampling rate considerations.
Thus, a need exists for a simple and effective method of suppressing on-off interference from bandspread communication signals. Accordingly, an object of the present invention is to provide a method and apparatus for suppressing on-off interference from bandspread communication signals whenever the interference dominates the signal being received. Another object of the present invention is to provide a method and apparatus for suppressing on-off interference from bandspread communication signals that is easily adapted to existing radio receivers. Still another object of the present invention is to provide a method and apparatus for suppressing any linear amplitude interference from bandspread communication signals.